The invention is directed to an apparatus for inducing hearing in persons primarily having sensory deafness, but also finds application where conventional hearing aids would normally be effective, but due to reduced hearing sensitivity, additional amplification or bone conduction is useless. Sensory deafness arises when the sensitivity of the cochlea in the internal ear is reduced due to, for example, loss of hair cells, or chemical changes in the perilymph. Reference is made to my U.S. Pat. No. 3,751,605 wherein the phenomena of sensory deafness is described in greater detail.
In the above patent, I disclosed a method for inducing the sensation of intelligible hearing in individuals with sensory deafness by direct electrical excitation of the auditory nerve endings distributed along the basilar membrane within the cochlea. In order to practice the method a bipolar electrode is positioned within the lower scala of the cochlea and a single channel excitation signal is conducted to the electrodes to simulate the naturally generated auditory electric field. The bipolar electrode comprises a pair of conductors running the length of a retaining base member which is resilient and shaped to conform to the inner surface of the lower scala.
While the above method successfully induces the sensation of intelligible hearing in individuals having sensory deafness, the apparatus utilized has produced limited success.
Other attempts at correcting sensory deafness have been directed to implants of electrodes directly into the auditory nerve as opposed to use of an intra-cochlear electrode. Reference is made to Kissiah, Jr. U.S. Pat. No. 4,063,048 wherein single electrodes are implanted in selected locations of the auditory nerve (bypassing a non-functioning cochlea altogether) and are supplied with digitized signal components derived from bandpass filters which filter the electrical analog of the external audio signal into several frequency bands. In Kissiah, it is stated that digitized signals are required because the cochlea does not have the capability to convert analog voltages to digital pulses for stimulation of acoustic nerve endings and, therefore, when analog signals are used the result is severe distortion and lack of fidelity. In addition, it is stated that a major fault of prior devices was the failure to maintain insulation between one implanted electrode and another, especially in intra-cochlear type electrodes where the surrounding cochlear fluid is a non-insulating liquid, thus resulting in stimulation of both intended and non-intended groups of nerve endings.
Contrary to the teachings of Kissiah, and in accordance with the teachings of the present invention, it has been discovered that an apparatus which supplies multichannel analog electrical signals through an intra-cochlear electrode array to nerve endings in the basilar membrane, can be used successfully to induce sensations of intelligible hearing in individuals having sensory deafness.
Experimental results confirm a marked improvement in speech discrimination over the prior art through use of the present invention. When subjects having a single channel electrode apparatus, as described in my prior patent, were compared against those having the multichannel electrode array system, using a standard Spondee word list, the latter scored 32% while the former did not score above chance. In subsequent experiments with the multichannel electrode array system scores of greater than 50% have been achieved with a standard Spondee word list.
It is believed that these results indicate, for the first time, an unequivocal demonstration of marked improvement of discrimination of speech as a result of using a hearing aid system of the present type.